1. Field of the Invention
The present invention relates to a biodegradable complex fiber and a method for producing the fiber, and more particularly, to a biodegradable complex fiber which can be widely used as fishing materials, e.g., fishing lines and fish nets; agricultural materials, e.g., insect or bird nets and vegetation nets; cloth fibers and non-woven fibers for articles for everyday life, e.g., disposable women's sanitary items, masks, wet tissues, underwear, towels, handkerchiefs, kitchen towels and diapers; and medical supplies, e.g., operating sutures which are not removed, operating nets and suture-reinforcing materials; and which does not pollute the environment. The present invention also relates to a method for producing the biodegradable fiber.
2. Description of Related Art
As polymer materials used for fishing lines, fish nets, agricultural nets, articles for everyday life or the like, those comprising, for example, a polyamide, polyester, polyvinyl chloride or polyolefin have been used. These polymer materials are resistant to degradation and hence have the problem that the environment is polluted when the above products are left in the environment after they are used. In order to solve this problem, these products must be subjected to treatment such as incineration, recovery and recycling after being used. However, such treatment imposes considerable costs. Moreover, many used products cannot be recovered and are left in the environment, causing environmental disruption.
Among methods used to solve such a problem, there is a method utilizing a polymer material which is easily degraded by microorganisms present in the natural world. For example, surgical sutures comprising poly-.epsilon.-caprolactone and monofilaments comprising poly-.beta.-propiolactone are disclosed in Japanese Patent Application Laid-Open (JP-A) Nos. H1-175855 and H5-78912 respectively. Poly-.epsilon.-caprolactone and poly-.beta.-propiolactone, however, have melting temperatures as low as about 60.degree. C. and about 97.degree. C., respectively, giving rise to the problem that methods of using these compounds are limited.
Also, JP-A No. H5-93316 discloses a microorganism-degradable complex fiber using poly-.epsilon.-caprolactone and/or poly-.beta.-propiolactone as the core component and poly(.beta.-hydroxyalkanoate) or a copolymer thereof as the shell component. However, the melting temperatures of poly-.epsilon.-caprolactone and poly-.beta.-propiolactone are about 60.degree. C. and about 97.degree. C. Therefore, in the case of using these compounds as fibers, deterioration of the strength of the fibers cannot be avoided when the operating temperature exceeds 100.degree. C. or the temperature partly exceeds 100.degree. C. by frictional heat.
As for an example of microorganism-degradable fibers having high melting temperature, surgical sutural materials comprising polylactic acid and a copolymer thereof are disclosed in JP-A No. S45-31696. However, such a fiber has insufficient strength and even though it can be made into a monofilament, the resulting monofilament is very hard so that it can be tied only with difficulty. Also its degradation is slow and cannot be controlled. As for polyglycolic acid type and polylactide type fibers, these fibers are commercially available as sutures. These fibers are, however, sensitive to moisture and tend to deteriorate. Also, these fibers are hard and this tends to limit their application. Moreover, they have a biological compatibility problem. For instance, when they are used as a suture for blood vessels, they can be said to be unsuitable because thrombi tend to be produced and adhesions of tissue are caused.
Biodegradable polyester fibers using random copolymer polyesters containing a 3-hydroxybutyric acid unit produced by microorganisms are disclosed in Biomaterials, 1987, Vol 8, 129. These so-called poly(3-hydroxybutyric acid) groups, i.e. homopolymers or copolymers containing less than 20% of other monomer units (hereinafter simply referred to as poly(3-hydroxybutyric acid)) are known to be degraded very well by bacteria which exist under the ground and in water in a large number. Also, they are used in applications, such as non-woven fabrics for preventing adhesions of tissue after surgical operations, because of their excellent biological compatibility. However, when they are made into fibers, the spinning and drawing of these fibers are found to be difficult, giving rise to the problem that high strength fibers cannot be obtained. For example, it is reported that after a poly(3-hydroxybutyric acid) produced by microorganisms are melted and extruded in a melt spinning step, they are deformed rubber-wise in a stage of drawing them into filaments when they are not crystallized whereas when they are highly crystallized, they are brittle-fractured at any temperature or even if any stress is applied, with the result that the spun filaments are brittle and hence have very low strength (Elsevier Applied Science, London, pp33-43, 1988).